Almost all epidemiologic studies of breast cancer have been conducted in Western populations with homogeneous and high exposure to certain hypothetical risk factors; the narrow range of exposure has substantially reduced efficiency in these studies. A population based case-control study is proposed among Chinese women in Shanghai where the incidence rate of breast cancer has increased dramatically in the past two decades. The investigators state that the unique lifestyle pattern and diverse exposures in this population will facilitate a rigorous examination of some important etiologic hypotheses that cannot be adequately addressed among women in the US. The primary aims of this study are to investigate the associations of breast cancer with dietary fat intake across the range 14% to 36% of calories (median of lowest to highest quintile), nutritional status during adolescence (including adolescent diet and certain anthropometries), estrogen metabolic pattern, body mass and fat distribution, oral contraceptive use, induced abortions, and breast feeding. The study will include a total of 1200 incident breast cancer cases aged 25-64 and an equal number of controls (frequency matched to cases on age) randomly selected from the general population in Shanghai. Cases will be identified through two well tried rapid case finding systems and the population based Shanghai Cancer Registry. In person interview data, anthropometric measurements and fasting blood and urine will be collected. For 500 pairs of cases and controls, urine samples will be assayed for levels of 2 and 16-alpha hydroxyestrones. Tumor tissue blocks will be collected for assays of estrogen and progesterone receptors to assess whether receptor defined subgroups of breast cancer are etiologically distinct. The remaining biospecimens will be stored at -70C for future molecular epidemiologic studies of serum organochlorine pesticides, somatic mutations in tumor tissue, genotypes of HRAS and certain metabolizing enzymes using DNA from white blood cells and other biomarkers. The investigators state that this study will provide valuable information on the interplay of hormonal, genetic, dietary, environmental, and lifestyle factors in the development of breast cancer. They further state that it will also create a valuable resource for future studies of genetic factors and gene-environmental interaction in the etiology of breast cancer.